1. Field of the Invention
The present invention relates generally to polarization maintaining and single polarization optical fibers that include a plurality of rods and/or a plurality of air holes in the cladding area.
2. Technical Background
Optical fiber has become a favorite medium for telecommunications due to its high capacity and immunity to electrical noise. Polarization maintaining (PM) fibers and single polarization (SP) fibers have been widely used to produce linear polarized output in optical systems. These fibers are useful for ultra-high speed transmission systems and are also utilized as couplers fiber for use with, and connection to, optical components (e.g., lasers, EDFAs, optical instruments, interferometric sensors, and gyroscopes). The polarization maintaining fibers and single polarization fibers can either be active, which means that they are rare earth doped in the fiber core, or passive, which means no rare earth dopants are involved. The passive PM fiber is useful for advanced coherent-communication systems, whereas the active PM fiber allows a fiber laser to have higher brightness, which is one of the two most important factors (the other being power) in laser output performance.
Polarization retaining fibers (sometimes referred to as a polarization maintaining fibers) can maintain the input polarizations on two generally-orthogonal axes. These fibers are not single polarization fibers. A common polarization maintaining fiber includes stress birefringence members and includes, as shown in FIG. 1A, a circular core 12′ surrounded by an cladding region 14′. Core 12′ and the cladding region 14′ are formed of conventional materials employed in the formation of optical waveguides. The refractive index of the core material is greater than that of the cladding material.
In FIG. 1A, diametrically opposed relative to core 12′, are two stress-inducing regions 13′ formed of a glass material having a Thermal Coefficient of Expansion (TCE) different from that of cladding material 14′. When such a fiber is drawn, the longitudinally-extending stress-inducing regions 13′ and the cladding region will shrink different amounts, whereby regions 13′ will be put into a state of tension or compression strain. Strain induced birefringence (otherwise referred to a stress-induced birefringence) is imparted in the fiber and thereby reduces coupling between the two orthogonally polarized fundamental modes. It should be recognized that such fibers including these stress-inducing regions 13′ do not provide single polarization properties.
Conventionally, PM fiber is typically made by drilling two-holes, then filled with stress-rods, around core in the fiber preform, or by a chemical boron-soot etching process during core preform laydown stage in the MCVD ‘Bow-tie’ PM fiber making. Recent development in advanced fiber applications requires fiber with more complex structures. These structures are more difficult to obtain with the conventional manufacturing methods.
Single polarization fiber propagates one, and only one, of two orthogonally polarized polarizations within a single polarization band while suppressing the other polarization by dramatically increasing its transmission loss. Single polarization fibers with a plurality of air-holes are known, however such fibers have a high relative refractive index core delta which makes it difficult to achieve large mode field diameter. It has, therefore, been an area of ongoing development to obtain an optical fiber that will have single polarization performance and also has a large mode field area.
On the other hand, another ongoing development is to obtain an optical fiber that is single polarization with sufficiently wide operating bandwidth. This will make it possible for such fibers to be used in greater range of applications which require wider range of wavelength operating window and it will also make the deployment of such fibers to a specific application easier as one fiber can fit the need for applications with quite different operating wavelengths. Another desired feature is to have a single polarization fiber with a large mode area that is comparable or larger than conventional fibers such as Panda type polarization maintaining fibers. However, in order to achieve wide single polarization bandwidth (by using only a plurality of air holes), high fiber core delta is required to generate large enough index contrast for high birefringence. Therefore, to maintaining single mode operation, the fiber core must be small, which limits the use of single polarization fibers for high power application.